Method and device for encoding and decoding a hdr picture and a ldr picture

ABSTRACT

The present disclosure generally relates to a method for encoding a HDR picture and a first LDR picture, the method comprising obtaining a second LDR picture responsive to a ratio between the HDR picture and a backlight picture and encoding the first LDR picture and the second LDR picture by predicting one of the first and second LDR pictures by the other one of the first and second LDR pictures. The method wherein it further comprises, before encoding, adjusting the first LDR picture responsive to the backlight picture. The disclosure relates also to a method and device for decoding an LDR picture providing a lower dynamic range depiction of the picture content of an HDR picture.

1. FIELD

The present disclosure generally relates to picture/video encoding anddecoding. In particular, the technical field of the present disclosureis related to encoding/decoding of a HDR picture whose pixels valuesbelong to a high-dynamic range together with a LDR picture providing alower dynamic range depiction of the picture content of the HDR picture.

The present disclosure further relates to method and device forencoding/decoding a sequence of HDR pictures, computer readableprograms, processor readable medium and non-transitory storage medium.

2. BACKGROUND

The present section is intended to introduce the reader to variousaspects of art, which may be related to various aspects of the presentdisclosure that are described and/or claimed below. This discussion isbelieved to be helpful in providing the reader with backgroundinformation to facilitate a better understanding of the various aspectsof the present disclosure.

Low-Dynamic-Range pictures (LDR pictures) are pictures whose luminancevalues are represented with a limited number of bits (most often 8 or10). This limited representation does not allow correct rendering ofsmall signal variations, in particular in dark and bright luminanceranges. In high-dynamic range pictures (HDR pictures), the signalrepresentation is extended in order to maintain a high accuracy of thesignal over its entire range. In HDR pictures, pixel values are usuallyrepresented in floating-point format (either 32 bits or 16 bits for eachcomponent, namely float or half-float), the most popular format beingopenEXR half-float format (16 bits per RGB component, i.e. 48 bits perpixel) or in integers with a long representation, typically at least 16bits.

A typical approach for encoding an HDR picture is to reduce the dynamicrange of the picture in order to encode the picture by means of atraditional encoding scheme (initially configured to encode LDRpictures).

According to a first approach, a tone-mapping operator is applied to theinput HDR picture and the tone-mapped picture is then encoded by meansof a traditional 8-10 bits-depth encoding scheme such as JPEG/JPEG200 orMPEG-2, H.264/AVC for sequences of HDR pictures (“The H.264 AdvancedVideo Compression Standard”, second edition, Iain E. Richardson, Wiley).Then, an inverse tone-mapping operator is applied to the decoded pictureand a residual picture is calculated between the input picture and thedecoded and inverse-tone-mapped picture. Finally, the residual pictureis encoded by means of a second traditional 8-10 bits-depth encoderscheme.

This first approach is backward compatible in the sense that a LDRpicture may be decoded and displayed by means of a traditionalapparatus.

This first approach uses two encoding schemes and limits the dynamicrange of the input picture to be twice the dynamic range of atraditional encoding scheme (16-20 bits). Moreover, such approach leadssometimes to a LDR picture with a weaker correlation with the input HDRpicture. This leads to low predictive-coding performance of the pictureor sequence of pictures.

According to a second approach, a backlight picture is determined fromthe luminance component of an input HDR picture. A residual picture isthen obtained by dividing the input HDR picture by the backlight pictureand both the backlight picture and the residual picture are directlyencoded.

FIG. 1 shows an example of this second approach for encoding a HDRpicture (more details for example in WO2013/102560).

In step 100, a module IC obtains the luminance component L andpotentially at least one color component C(i) of a HDR picture I to beencoded. The HDR picture I may belong to a sequence of HDR pictures.

For example, when the HDR picture I belongs to the color space (X,Y,Z),the luminance component L is obtained by a transform f(.) of thecomponent Y, e.g. L=f(Y).

When the HDR picture I belongs to the color space (R,G,B), the luminancecomponent L is obtained, for instance in the 709 gamut, by a linearcombination which is given by:

L=0.2127.R+0.7152.G+0.0722.B

In step 101, a module BAM determines a backlight picture Bal from theluminance component L of the HDR picture I.

In step 102, the data needed to determine the backlight picture Bal,output from step 101, are encoded by means of an encoder ENC2 and addedin a bitstream F2 which may be stored on a local or remote memory and/ortransmitted through a communication interface (e.g. to a bus or over acommunication network or a broadcast network).

In step 103, a LDR picture LDR2 is obtained from a ratio between the HDRpicture and the backlight picture Bal.

More precisely, the luminance component L and potentially each colourcomponent C(i) of the picture I, obtained from the module IC, is dividedby the backlight picture Bal. This division is done pixel per pixel.

For example, when the components R, G or B of the HDR picture I areexpressed in the color space (R,G,B), the components R_(LDR2), G_(LDR2)and B_(LDR2) are obtained as follows:

R _(LDR2=) R/Bal, G _(LDR2=) G/Bal, B _(LDR2=) B/Bal.

For example, when the components X, Y or Z of the HDR picture I areexpressed in the color space (Y,Y,Z), the components R_(LDR2), Y_(LDR2)and Z_(LDR2) are obtained as follows:

X _(LDR2=) X/Bal Y _(LDR2=) Y/Bal Z _(LDR2=) Z/Bal

In step 104, an operator TMO tone-maps the HDR picture I in order to geta LDR picture LDR1 having a lower dynamic range than the dynamic rangeof the HDR picture I.

Any specific tone-mapping operator may be used such as, for example, thetone-mapping operator defined by Reinhard may be used (Reinhard, E.,Stark, M., Shirley, P., and Ferwerda, J., \Photographic tonereproduction for digital pictures,” ACM Transactions on Graphics 21(July 2002), or Boitard, R., Bouatouch, K., Cozot, R., Thoreau, D., &Gruson, A. (2012). Temporal coherency for video tone mapping. In A. M.J. van Eijk, C. C. Davis, S. M. Hammel, & A. K. Majumdar (Eds.), Proc.SPIE 8499, Applications of Digital Picture Processing (p.84990D-84990D-10)).

In step 105, the LDR pictures LDR1 and LDR2 are encoded by means of apredictive encoder ENC1 in at least one bitstream F1. More precisely,the LDR picture LDR1 (or LDR2) is used as a reference picture to predictthe other LDR picture LDR2 (or LDR1). A residual picture is thusobtained by subtracting the prediction picture from the LDR picture andboth the residual picture and the prediction picture are encoded.

The bitstream F1 may be stored on a local or remote memory and/ortransmitted through a communication interface (e.g. on a bus or over acommunication network or a broadcast network).

This second approach is backward compatible in the sense that a LDRpicture LDR1 may be decoded and displayed by means of a traditionalapparatus and the HDR picture I may also be decoded and displayed bydecoding the LDR picture LDR2 and the data needed to determine a decodedversion of the backlight picture Bal.

This second approach leads sometimes to a LDR picture LDR1 with a weakercorrelation with the other LDR picture LDR2 because those two picturesare not obtained from the HDR picture I by using same means: one isobtained by dividing the HDR picture by the backlight picture Bal andthe other one is obtained by applying a tone-mapping operator. Thisleads to a sparse residual content having sometimes locally importantvalues (lighting artefacts), lowering thus the coding performance of thepicture or sequence of pictures.

3. SUMMARY

In light of the foregoing, aspects of the present disclosure aredirected to creating and maintaining relationships between data objectson a computer system. The following presents a simplified summary of thedisclosure in order to provide a basic understanding of some aspects ofthe disclosure. This summary is not an extensive overview of thedisclosure. It is not intended to identify key or critical elements ofthe disclosure. The following summary merely presents some aspects ofthe disclosure in a simplified form as a prelude to the more detaileddescription provided below.

The disclosure sets out to remedy some of the drawbacks of the prior artwith a method for encoding a HDR picture and a first LDR picture, themethod comprising:

-   -   obtaining a second LDR picture (LDR2) responsive to a ratio        between the HDR picture and a backlight picture (Bal);    -   encoding the first LDR picture and the second LDR picture (LDR2)        by predicting one of the first and second LDR pictures by the        other one of the first and second LDR pictures. The method        further comprises, before encoding, adjusting the first LDR        picture responsive to the backlight picture.

According to an embodiment, the first LDR picture is adjusted bymultiplying the first LDR picture by a coefficient value (CV1) whichdepends on the backlight picture (Bal).

Adjusting the first LDR picture responsive to the backlight pictureattenuate the lighting artefacts introduced, in the second LDR picture.Thus, introducing similar lighting artefacts in the first LDR pictureincreases the correlation between the two LDR pictures, decreases thedynamic of the residual and thus increases the predictive-codingperformance.

According to one of its other aspects, the disclosure relates to amethod for decoding a LDR picture providing a lower dynamic rangedepiction of the content of an HDR picture. The method obtaining the LDRpicture by a at least partially decoding a bitstream, is characterizedin that:

-   -   the decoded LDR picture is obtained by adjusting the obtained        LDR picture responsive to a backlight picture calculated from        the HDR picture.

Advantageously, introducing similar lighting artefacts in the LDRpicture is a process which is implemented, at the encoder side, byadjusting the obtained LDR picture responsive to a backlight picturecalculated from the HDR picture. Thus, at the decoder side, a inverseprocess is used for removing such lighting artifacts.

Thus, one advantage of the method is to reduce the coding cost of theLDR picture while maintaining the backward compatibility of theencoding/decoding method in the sense that the encoded LDR picture maybe decoded and displayed by means of a traditional apparatus.

The specific nature of the disclosure as well as other objects,advantages, features and uses of the disclosure will become evident fromthe following description of embodiments taken in conjunction with theaccompanying drawings.

4. BRIEF DESCRIPTION OF DRAWINGS

In the drawings, an embodiment of the present disclosure is illustrated.It shows:

FIG. 1 shows a block diagram of the steps of a method for encoding anHDR picture in accordance with prior art;

FIG. 2 shows a block diagram of the steps of a method for encoding anHDR picture in accordance with an embodiment of the disclosure;

FIG. 3 shows a block diagram of the steps of a method for decoding anHDR picture in accordance with an embodiment of the disclosure;

FIGS. 4-7 show block diagrams of a step of the method of FIG. 1 inaccordance with embodiments of the disclosure;

FIG. 8 shows an example of an architecture of a device in accordancewith an embodiment of the disclosure; and

FIG. 9 shows two remote devices communicating over a communicationnetwork in accordance with an embodiment of the disclosure.

Similar or same elements are referenced with the same reference numbers.

5. DESCRIPTION OF EMBODIMENTS

The present disclosure will be described more fully hereinafter withreference to the accompanying figures, in which embodiments of thedisclosure are shown. This disclosure may, however, be embodied in manyalternate forms and should not be construed as limited to theembodiments set forth herein. Accordingly, while the disclosure issusceptible to various modifications and alternative forms, specificembodiments thereof are shown by way of example in the drawings and willherein be described in detail. It should be understood, however, thatthere is no intent to limit the disclosure to the particular formsdisclosed, but on the contrary, the disclosure is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the disclosure as defined by the claims. Like numbers referto like elements throughout the description of the figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”,“comprising,” “includes” and/or “including” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof. Moreover, when an elementis referred to as being “responsive” or “connected” to another element,it can be directly responsive or connected to the other element, orintervening elements may be present. In contrast, when an element isreferred to as being “directly responsive” or “directly connected” toother element, there are no intervening elements present. As used hereinthe term “and/or” includes any and all combinations of one or more ofthe associated listed items and may be abbreviated as“/”.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement without departing from the teachings of the disclosure.

Although some of the diagrams include arrows on communication paths toshow a primary direction of communication, it is to be understood thatcommunication may occur in the opposite direction to the depictedarrows.

Some embodiments are described with regard to block diagrams andoperational flowcharts in which each block represents a circuit element,module, or portion of code which comprises one or more executableinstructions for implementing the specified logical function(s). Itshould also be noted that in other implementations, the function(s)noted in the blocks may occur out of the order noted. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently or the blocks may sometimes be executed in the reverseorder, depending on the functionality involved.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one implementation ofthe disclosure. The appearances of the phrase “in one embodiment” or“according to an embodiment” in various places in the specification arenot necessarily all referring to the same embodiment, nor are separateor alternative embodiments necessarily mutually exclusive of otherembodiments.

Reference numerals appearing in the claims are by way of illustrationonly and shall have no limiting effect on the scope of the claims.

While not explicitly described, the present embodiments and variants maybe employed in any combination or sub-combination.

The disclosure is described for encoding/decoding a picture but extendsto the encoding/decoding of a sequence of pictures (video) because eachpicture of the sequence is sequentially encoded/decoded as describedbelow.

FIG. 2 shows a block diagram of the steps of a method for encoding anHDR picture in accordance with an embodiment of the disclosure.

According to an embodiment of the step 101, illustrated in FIG. 4, amodule BI determines a backlight picture Ba as being a weighted linearcombination of shape functions ψ_(i) given by:

Ba=Σ_(i) a _(i)ψ_(i)   (1)

with a_(i) being weighting coefficients.

Thus, determining a backlight picture Ba from a luminance component Lconsists in finding optimal weighting coefficients (and potentially alsooptimal shape functions if not known beforehand) in order that thebacklight picture Ba fits the luminance component L.

There are many well-known methods to find the weighting coefficientsa_(i). For example, one may use a least mean square method to minimizethe mean square error between the backlight picture Ba and the luminancecomponent L.

The disclosure is not limited to any specific method to obtain thebacklight picture Ba.

It may be noted that the shape functions may be the true physicalresponse of a display backlight (made of LED's for instance, each shapefunction then corresponding to the response of one LED) or may be a puremathematical construction that fits the luminance component best.

According to this embodiment, the backlight picture Bal, output fromstep 101, is the backlight picture Ba given by equation (1).

According to an embodiment of the step 101, illustrated in FIG. 5, amodule BM modulates the backlight picture Ba (given by equation (1))with a mean luminance value L_(mean) of the HDR picture I obtained bythe means of a module HL.

According to this embodiment, the backlight picture Bal, output fromstep 101, is the modulated backlight picture.

According to an embodiment, the module HL is configured to calculate themean luminance value L_(mean) over the whole luminance component L.

According to an embodiment, the module HL is configured to calculate themean luminance value L_(mean) by

$L_{mean} = {E\left( L^{\beta} \right)}^{\frac{1}{\beta}}$

with β being a coefficient less than 1 and E(X) the mathematicalexpectation value (mean) of the luminance component L.

This last embodiment is advantageous because it avoids that the meanluminance value L_(mean) be influenced by a few pixels with extreme highvalues which usually leads to very annoying temporal mean brightnessinstability when the HDR picture I belongs to a sequence of HDRpictures.

The disclosure is not limited to a specific embodiment for calculatingthe mean luminance value L_(mean).

According to a variant of this embodiment, illustrated in FIG. 6, amodule N normalizes the backlight picture Ba (given by equation (1)) byits mean value E(Ba) such that one gets a mid-gray-at-one backlightpicture Ba_(gray) for the picture (or for all pictures if the HDRpicture I belongs to a sequence of HDR pictures):

${Ba}_{gray} = \frac{Ba}{E({Ba})}$

Then, the module BM is configured to modulate the mid-gray-at-onebacklight picture Ba_(gray) with the luminance average value L_(mean) ofthe luminance component L, by using the following relation

Ba_(mod)≈cst_(mod).L_(mean) ^(α).Ba_(gray)   (2)

with cst_(mod) being a modulation coefficient and a being anothermodulation coefficient less than 1, typically ⅓.

A low-spatial-frequency version L_(lf) of the luminance component L maybe obtained, for example, by subsampling the luminance component L.

According to this variant, the backlight picture Bal, output from step101, is the modulated backlight picture Ba_(mod) given by equation (2).

It may be noted that the modulation coefficient cst_(mod) is tuned toget a good looking brightness for the residual picture and highlydepends on the process to obtain the backlight picture. For example,cst_(mod)≈1.7 for a backlight picture obtained by least means squares.

Practically, by linearity, all operations to modulate the backlightpicture apply to the backlight coefficients a_(i) as a correcting factorwhich transforms the coefficients a_(i) into new coefficients ã_(l) suchthat one gets

${Ba}_{mod} = {\sum\limits_{i}\; {{\overset{\sim}{a}}_{\iota}\psi_{i}}}$

According to an embodiment of the step 101, illustrated in FIG. 7, thebacklight picture Bal is determined from a processed version L′ of theluminance component L. Such a processed version L′ is, for example,determined by means of a module SQR which calculates the squared root ofthe luminance component L. The resulting version of the luminancecomponent L is then sub-sampled in order that each value of thesub-sampled version of the luminance component L represents theluminance of a group of pixels of the HDR picture I.

Note that the processed version L′ of the luminance component L may alsobe used rather than the luminance component L in the embodimentsillustrated in FIGS. 4, 5 and 6.

As mentioned in the introductive part, the data needed to determine thebacklight picture Bal, output from step 101, are encoded by means of anencoder ENC2 and added in a bitstream F2.

For example, the data to be encoded are limited to the weightingcoefficients a_(i) or ã_(l) when known non-adaptive shape functions areused, but the shape functions ψ_(i) may also be a priori unknown andthen encoded in the bitstream F2, for instance in a case of a somewhatoptimal mathematical construction for better fitting. So, all theweighting coefficients a_(i) or ã_(l) (and potentially shape functionsψ_(i)) are encoded in the bitstream F2.

Advantageously, the weighting coefficients a_(i) or ã_(l) are quantizedbefore being encoded in order to reduce the size of the bitstream F2.

The bitstreams F1 and F2 may be either separate bitstream or parts of asame bitstream.

In step 200 (FIG. 2), a module LMA adjusts the picture LDR1 responsiveto the backlight picture Bal in order to attenuate the lightingartefacts.

According to an embodiment of the step 200, the LDR picture LDR1 isadjusted by dividing the LDR picture LDR1 by a coefficient value CV1which depends on the backlight picture Bal.

Mathematically speaking, such adjusting can be given by:

LDR1′(x,y)=int(LDR1(x,y)/CV1)

where (x,y) is the spatial position of a pixel of the picture and int( )is the integer value with the same bit-depth (e.g. 10 bits).

According to an embodiment of the step 200, the coefficient value CV1 isproportional to the pixel values of the backlight picture Bal:

CV1=norm(Bal(x,y))*2

where norm means the normalization of the backlight picture Bal (between0 and 1).

FIG. 3 shows a block diagram of the steps of a method for decoding anHDR picture in accordance with an embodiment of the disclosure.

In step 301, a decoded version {circumflex over (B)}a of the backlightpicture is obtained by decoding at least partially the bitstream F2 bymeans of a decoder DEC2.

As explained before, some data needed to obtain the backlight picture,output of step 101, have been encoded (step 102) and then obtained by atleast partially decoding the bitstream F2 which may have been storedlocally or received from a communication network.

Following the example given above, weighting coefficients â_(l) (andpotentially shape functions {circumflex over (ψ)}_(i)) are then obtainedas output of step 301.

Then, in step 302, a module BAG generates a decoded version {circumflexover (B)}a of the backlight picture from the weighting coefficientsâ_(l) and either some known non-adaptive shape functions or the shapefunctions {circumflex over (ψ)}_(l) by:

{circumflex over (B)} _(a)=Σ_(i) â _(l){circumflex over (ψ)}_(l)

In step 300, a first LDR picture L

′ and a second LDR picture

are obtained by at least a partial decoding of a bitstream F1 by meansof a decoder DEC1. The bitstream F1 may have been stored locally orreceived from a communication network.

In step 303, obtaining a decoded HDR picture Î responsive to a productof the second picture

by the backlight picture {circumflex over (B)}_(a).

In step 304, a module ILMA adjusts the first LDR picture L

′ responsive to the backlight picture {circumflex over (B)}_(a) in orderto get a decoded version of the LDR picture LDR1.

According to an embodiment of the step 304, the first LDR picture L

′ is adjusted by multiplying the first LDR picture L

′ by a coefficient value (CV2) which depends on the backlight picture{circumflex over (B)}_(a).

Mathematically speaking, such adjusting is given by:

L

(x, y)=int(L

′(x,y)*CV2)

where (x,y) is the spatial position of a pixel of the picture and intois the integer value with the same bit-depth (e.g. 10 bits).

According to an embodiment of the step 303, the coefficient CV2 isproportional to the pixel values of the backlight picture {circumflexover (B)}a given by:

CV2=norm

(x,y))*2

where norm is the normalization of backlight picture {circumflex over(B)}a signal (between 0 and 1).

The decoders DEC1 and DEC2 are configured to decode data which have beenencoded by the encoder ENC1 and ENC2, respectively.

The encoders ENC1 and ENC2 (and decoders DEC1 and DEC2) are not limitedto a specific encoder (decoder) but when an entropy encoder (decoder) isrequired, an entropy encoder such as a Huffmann coder, an arithmeticcoder or a context adaptive coder like CABAC used in H.264/AVC (“TheH.264 Advanced Video Compression Standard”, second edition, Iain E.Richardson, Wiley) or HEVC (B. Bross, W. J. Han, G. J. Sullivan, J. R.Ohm, T. Wiegand JCTVC-K1003, “High Efficiency Video Coding (HEVC) textspecification draft 9,” October 2012) is advantageous.

The encoders ENC1 and ENC2 (and decoders DEC1 and DEC2) are not limitedto a specific encoder which may be, for example, an picture/video coderlike JPEG, JPEG2000, MPEG2, h264/AVC or HEVC.

Preferably, the ENC2 and DEC2 are lossless encoding/decoding scheme.

As described above, the LDR pictures LDR1′ and LDR2 are encoded in asame bitstream F1. The encoder ENC1 may then conform to MVC (ISO/IEC14996-10 annex H or ITU-T H.264 annex H).

But the LDR picture LDR1′ and LDR2 may also be encoded in two separatebitstreams F1.

The present disclose relates also to a method for encoding a sequence ofHDR pictures wherein each HDR picture of the sequence of HDR pictures isencoded according to a method described in relation with FIGS. 2, 4-7.

The present disclose relates also to a method for decoding a sequence ofHDR pictures wherein each HDR picture of the sequence of HDR pictures isdecoded according to a method described in relation with FIG. 3.

On FIGS. 2-7, the modules are functional units, which may or not be inrelation with distinguishable physical units. For example, these modulesor some of them may be brought together in a unique component orcircuit, or contribute to functionalities of a software. A contrario,some modules may potentially be composed of separate physical entities.The apparatus which are compatible with the disclosure are implementedusing either pure hardware, for example using dedicated hardware suchASIC or FPGA or VLSI, respectively “Application Specific IntegratedCircuit”, “Field-Programmable Gate Array”, “Very Large ScaleIntegration”, or from several integrated electronic components embeddedin a device or from a combination of hardware and software components.

FIG. 8 represents an exemplary architecture of a device 800 which may beconfigured to implement a method described in relation with FIGS. 2,4-7.

Device 800 comprises following elements that are linked together by adata and address bus 801:

-   -   a microprocessor 802 (or CPU), which is, for example, a DSP (or        Digital Signal Processor);    -   a ROM (or Read Only Memory) 803;    -   a RAM (or Random Access Memory) 804;    -   an I/O interface 805 for reception of data to transmit, from an        application; and    -   a battery 806

According to a variant, the battery 806 is external to the device. Eachof these elements of FIG. 8 is well-known by those skilled in the artand won't be disclosed further. In each of mentioned memory, the word“register” used in the specification can correspond to area of smallcapacity (some bits) or to very large area (e.g. a whole program orlarge amount of received or decoded data). ROM 803 comprises at least aprogram and parameters. Algorithm of the methods according to thedisclosure is stored in the ROM 803. When switched on, the CPU 802uploads the program in the RAM and executes the correspondinginstructions.

RAM 804 comprises, in a register, the program executed by the CPU 802and uploaded after switch on of the device 800, input data in aregister, intermediate data in different states of the method in aregister, and other variables used for the execution of the method in aregister.

The implementations described herein may be implemented in, for example,a method or a process, an apparatus, a software program, a data stream,or a signal. Even if only discussed in the context of a single form ofimplementation (for example, discussed only as a method or a device),the implementation of features discussed may also be implemented inother forms (for example a program). An apparatus may be implemented in,for example, appropriate hardware, software, and firmware. The methodsmay be implemented in, for example, an apparatus such as, for example, aprocessor, which refers to processing devices in general, including, forexample, a computer, a microprocessor, an integrated circuit, or aprogrammable logic device. Processors also include communicationdevices, such as, for example, computers, cell phones, portable/personaldigital assistants (“PDAs”), and other devices that facilitatecommunication of information between end-users.

According to an embodiment, the device 800 may be further configured toimplement a method described in relation with FIG. 3. Thus, the device800 may be either an encoder, a decoder or both.

According to a specific embodiment of encoding or encoder, the HDRpicture I is obtained from a source. For example, the source belongs toa set comprising:

-   -   a local memory (803 or 804), e.g. a video memory or a RAM (or        Random Access Memory), a flash memory, a ROM (or Read Only        Memory), a hard disk;    -   a storage interface (805), e.g. an interface with a mass        storage, a RAM, a flash memory, a ROM, an optical disc or a        magnetic support;    -   a communication interface (805), e.g. a wireline interface (for        example a bus interface, a wide area network interface, a local        area network interface) or a wireless interface (such as a IEEE        802.11 interface or a Bluetooth® interface); and    -   an picture capturing circuit (e.g. a sensor such as, for        example, a CCD (or Charge-Coupled Device) or CMOS (or        Complementary Metal-Oxide-Semiconductor)).

According to different embodiments of the decoding or decoder, thedecoded HDR picture Î or the decoded picture L

is sent to a destination; specifically, the destination belongs to a setcomprising:

-   -   a local memory (803 or 804), e.g. a video memory or a RAM, a        flash memory, a hard disk;    -   a storage interface (805), e.g. an interface with a mass        storage, a RAM, a flash memory, a ROM, an optical disc or a        magnetic support;    -   a communication interface (805), e.g. a wireline interface (for        example a bus interface (e.g. USB (or Universal Serial Bus)), a        wide area network interface, a local area network interface, a        HDMI (High Definition Multimedia Interface) interface) or a        wireless interface (such as a IEEE 802.11 interface, WiFi ® or a        Bluetooth® interface); and    -   a display.

According to different embodiments of encoding or encoder, the bitstreamF1 and/or F2 are sent to a destination. As an example, one of bitstreamF1 and F2 or both bitstreams F1 and F2 are stored in a local or remotememory, e.g. a video memory (804) or a RAM (804), a hard disk (803). Ina variant, one or both bitstreams are sent to a storage interface (805),e.g. an interface with a mass storage, a flash memory, ROM, an opticaldisc or a magnetic support and/or transmitted over a communicationinterface (805), e.g. an interface to a point to point link, acommunication bus, a point to multipoint link or a broadcast network.

According to different embodiments of decoding or decoder, the bitstreamF1 and/or F2 is obtained from a source. Exemplarily, the bitstream isread from a local memory, e.g. a video memory (804), a RAM (804), a ROM(803), a flash memory (803) or a hard disk (803). In a variant, thebitstream is received from a storage interface (805), e.g. an interfacewith a mass storage, a RAM, a ROM, a flash memory, an optical disc or amagnetic support and/or received from a communication interface (65),e.g. an interface to a point to point link, a bus, a point to multipointlink or a broadcast network.

According to different embodiments, device 800 being configured toimplement an encoding method described in relation with FIGS. 2, 4-7,belongs to a set comprising:

-   -   a mobile device;    -   a communication device;    -   a game device;    -   a tablet (or tablet computer);    -   a laptop;    -   a still picture camera;    -   a video camera;    -   an encoding chip;    -   a still picture server; and    -   a video server (e.g. a broadcast server, a video-on-demand        server or a web server).

According to different embodiments, device 800 being configured toimplement a decoding method described in relation with FIG. 3, belongsto a set comprising:

-   -   a mobile device;    -   a communication device;    -   a game device;    -   a set top box;    -   a TV set;    -   a tablet (or tablet computer);    -   a laptop;    -   a display and    -   a decoding chip.

According to an embodiment illustrated in FIG. 9, in a transmissioncontext between two remote devices A and B over a communication networkNET, the device A comprises means which are configured to implement amethod for encoding an picture as described in relation with the FIGS.2, 4-7 and the device B comprises means which are configured toimplement a method for decoding as described in relation with FIG. 3.

According to a variant of the disclosure, the network is a broadcastnetwork, adapted to broadcast still pictures or video pictures fromdevice A to decoding devices including the device B.

Implementations of the various processes and features described hereinmay be embodied in a variety of different equipment or applications,particularly, for example, equipment or applications. Examples of suchequipment include an encoder, a decoder, a post-processor processingoutput from a decoder, a pre-processor providing input to an encoder, avideo coder, a video decoder, a video codec, a web server, a set-topbox, a laptop, a personal computer, a cell phone, a PDA, and othercommunication devices. As should be clear, the equipment may be mobileand even installed in a mobile vehicle.

Additionally, the methods may be implemented by instructions beingperformed by a processor, and such instructions (and/or data valuesproduced by an implementation) may be stored on a computer readablestorage medium. A computer readable storage medium can take the form ofa computer readable program product embodied in one or more computerreadable medium(s) and having computer readable program code embodiedthereon that is executable by a computer. A computer readable storagemedium as used herein is considered a non-transitory storage mediumgiven the inherent capability to store the information therein as wellas the inherent capability to provide retrieval of the informationtherefrom. A computer readable storage medium can be, for example, butis not limited to, an electronic, magnetic, optical, electromagnetic,infrared, or semiconductor system, apparatus, or device, or any suitablecombination of the foregoing. It is to be appreciated that thefollowing, while providing more specific examples of computer readablestorage mediums to which the present principles can be applied, ismerely an illustrative and not exhaustive listing as is readilyappreciated by one of ordinary skill in the art: a portable computerdiskette; a hard disk; a read-only memory (ROM); an erasableprogrammable read-only memory (EPROM or Flash memory); a portablecompact disc read-only memory (CD-ROM); an optical storage device; amagnetic storage device; or any suitable combination of the foregoing.

The instructions may form an application program tangibly embodied on aprocessor-readable medium.

Instructions may be, for example, in hardware, firmware, software, or acombination. Instructions may be found in, for example, an operatingsystem, a separate application, or a combination of the two. A processormay be characterized, therefore, as, for example, both a deviceconfigured to carry out a process and a device that includes aprocessor-readable medium (such as a storage device) having instructionsfor carrying out a process. Further, a processor-readable medium maystore, in addition to or in lieu of instructions, data values producedby an implementation.

As will be evident to one of skill in the art, implementations mayproduce a variety of signals formatted to carry information that may be,for example, stored or transmitted. The information may include, forexample, instructions for performing a method, or data produced by oneof the described implementations. For example, a signal may be formattedto carry as data the rules for writing or reading the syntax of adescribed embodiment, or to carry as data the actual syntax-valueswritten by a described embodiment. Such a signal may be formatted, forexample, as an electromagnetic wave (for example, using a radiofrequency portion of spectrum) or as a baseband signal. The formattingmay include, for example, encoding a data stream and modulating acarrier with the encoded data stream. The information that the signalcarries may be, for example, analog or digital information. The signalmay be transmitted over a variety of different wired or wireless links,as is known. The signal may be stored on a processor-readable medium.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. For example,elements of different implementations may be combined, supplemented,modified, or removed to produce other implementations. Additionally, oneof ordinary skill will understand that other structures and processesmay be substituted for those disclosed and the resulting implementationswill perform at least substantially the same function(s), in at leastsubstantially the same way(s), to achieve at least substantially thesame result(s) as the implementations disclosed. Accordingly, these andother implementations are contemplated by this application.

1. method for encoding a HDR picture whose pixels values belong to ahigh-dynamic range, and a first LDR picture, whose pixels values belongto a lower-dynamic range, the method comprising: obtaining a second LDRpicture from a ratio between the HDR picture and a backlight picturedetermined from the luminance of the HDR picture; encoding the first LDRpicture and the second LDR picture by predicting one of the first andsecond LDR pictures by the other one of the first and second LDRpictures; wherein it further comprises, before encoding, multiplying thefirst LDR picture by a coefficient value which depends on the backlightpicture.
 2. The method of claim 1, wherein the coefficient value isproportional to the pixel values of the backlight picture.
 3. The methodof claim 1, wherein data needed to determine the backlight picture arelossless encoded.
 4. A method for decoding a LDR picture providing alower dynamic range depiction of the content of an HDR picture, themethod obtaining the LDR picture by a at least partially decoding abitstream, wherein: the decoded LDR picture is obtained by multiplyingthe obtained LDR picture responsive of a backlight picture calculatedfrom the HDR picture by a coefficient value which depends on thebacklight picture.
 5. The method according to claim 4, wherein thecoefficient value is proportional to the pixel values of the backlightpicture.
 6. The method for encoding a sequence of HDR pictures, whereineach HDR picture of the sequence of HDR pictures is encoded according toa method conforms to claim
 1. 7. The method for decoding a sequence ofHDR pictures, wherein each HDR picture of the sequence of HDR picturesis decoded according to a method conforms to claim
 4. 8. A device forencoding a HDR picture, whose pixels values belong to a high-dynamicrange, together with a first LDR picture whose pixels values belong to alower-dynamic range, the device comprises a processor configured to:obtain a second LDR picture from a ratio between the HDR picture and abacklight picture determined from the luminance of the HDR picture;encode the first LDR picture and the second LDR picture by predictingone of the first and second LDR pictures by the other one of the firstand second LDR pictures; wherein the processor is further configured to,before encoding, multiply the first LDR picture by a coefficient valuewhich depends on the backlight picture (Bal).
 9. A device for decoding aLDR picture providing a lower dynamic range depiction of the content ofan HDR picture, the device comprising a processor configured to obtainthe LDR picture by a at least partially decoding a bitstream, the devicecomprising the processor is further configured to: obtain the decodedLDR picture by multiplying the LDR picture by a coefficient value whichdepends on a backlight picture determined from the luminance of the HDRpicture;
 10. A computer program product comprising program codeinstructions to execute the steps of the encoding method according toclaim 1 when this program is executed on a computer.
 11. A computerprogram product comprising program code instructions to execute thesteps of the decoding method according to claim 4 when this program isexecuted on a computer.
 12. A processor readable medium having storedtherein instructions for causing a processor to perform at least thesteps of the encoding method according to claim
 1. 13. A processorreadable medium having stored therein instructions for causing aprocessor to perform at least the steps of the decoding method accordingto claim
 4. 16. Non-transitory storage medium carrying instructions ofprogram code for executing steps of a method according to claim 1, whensaid program is executed on a computing device.